Abstract.1. The acceptability of sucrose solutions to Formica lugubris Zett. in the field was tested by offering droplets of known concentrations to ants tending aphids.2. A probit model fitted to the data allowed the computation of the Median Effective Concentration (ED50) and its confidence limits.3. In spring and early summer ED50was about 0.15 M sucrose, but the ED50rose in 1 week at the end of June from 0.2 M to 0.9 M. The change was simultaneous on pine and birch trees. The ED50fell gradually to about 0.4 M in early November.4. The increase in ED50is related to the production of the summer apterae in the aphids tended, Cinara pini (L.) on pines and Symydobius oblongus (von Heyden) on birch. This increases the ‘quality’ of food resource so that low sucrose concentrations can no longer compete for ant attendance.5. The increase in ED50may limit the attendance of wood‐ants at the nectaries of bracken, Pteridium aquilinum (L.) Kuhn. and at colonies of some other aphid species which are attended only in early s